For the agroecological transition, the rhizosphere is a critical interface for plants to acquire resources and to enhance plant health with limited inputs. In the present study, we developed a new indicator to estimate and monitor the intensity of plant-soil-microbiota interactions under field conditions. A Rhizosphere Effect Indicator (REI) was calculated by comparing individual and aggregated variables of bulk soil to those of the rhizosphere (i.e. soil enzyme activities and nutrient fluxes) every 2 weeks in the fields of three farms along a crop-diversification gradient. The diversity and structure of microbial communities in bulk soil and the rhizosphere in each field were assessed at flowering. The REI revealed statistically distinct dynamics and intensities of rhizosphere functioning among the three farms. Soil enzyme activities contributed more to the rhizosphere effect than nitrate and phosphate fluxes. Molecular analysis distinguished the two soil compartments and all crop-diversification levels. An integrated REI that provided a single value monitored the rhizosphere effect reliably. Methodological limits due to sampling of the rhizosphere under field conditions must be overcome to develop the REI, but the REI can serve as a complementary tool to traditional soil analysis for agroecological cropping system design and evaluation.